Noise suppression means for communication receiving apparatus



May 30, 1950 J.- E. WERNER ETAL NOISE SUPPRESSION MEANS FOR COMMUNICATION RECEIVING APPARATUS Filed Oct. 2s, 1946 mwm INIA

Il Il WMI@ ATTO RNEM R mm IHILIWS Patented May 30, 1950 TATES ,T OFFICE NOISE SUPPRESSION MEANS FOR COMMU- NICATION RECEIVING APPARATUS Application October 23, 1946, Serial No..705,094

2 Claims. (Cl. Z50-20) Our invention relates to noise suppressiony means for communication receiving apparatus, and more particularly to` noise suppression means for receiving apparatus` using frequency modulaion.

The receiving apparatusof communication systems, such as, railway train telephone systems, is generally retained` inl an active condition except during sending periods from the same location, and spurious energy picked` up during non-com munication periods may create` noise. Furthermore, in receiving apparatus for a relativelylow carrier frequency such as isusecll in railway train telephone systems4 of the inductive carrier type, the present day so called squelch circuits are not entirely satisfactory due to the invariably high noise energy level atthese low frequencies and the high gain required for the receiving apparatus.

Accordingly, a feature of our invention is the provision of improved noise suppression means for communication apparatus.

Another feature of our invention is the` provision of receiving apparatus of the type here involved incorporating` novel means Wherewith the output amplifier stage is desensitized during non-communication periods and is conditioned for its usable sensitivity Whencommunication energy of a preselected minimum value is picked, up.

Again, a feature of our invention is the provision of receiving apparatus` of the type here con.- templated incorporating a, novel squelch c ircuit arrangement having an automatic volume control.

Other features, objects` and advantages of our invention will appeal' as. the specification progresses.

To attain the foregoing objects, features and advantages of our` invention, we provide a controlled amplier tube and a noise responsive circuit network. The controlled amplifier tube is used to drive an audio frequency amplifier of the receiving apparatus and thus when the controlled tube is desensitized no energy is passed to the audio ampliiier with the result the loud-speakerV and other telephone receiversl in the output of the apparatus are silent.

The sensitivity of the controlled tube is governed through a voltage derived from a voltage divider network interposed in the anode circuit The noise responsive circuit network includes a noise amplifier` tube, av rectifier and an automatic volume control circuit together with the control or squelchV tube. The squelch tube is controlled by voltages derived from preselected terminals of the receiving apparatus' at points ahead ofthe demodulator. One of thel voltages is derived from the grid circuit of a limiter tube of the` apparatus, and asecond voltge is derived by taking the alternating current, noise voltage and amplifying it at the noise amplifier` and rectifying the amplified energy.

The automatic: volume control for the noise amplier is derived by combining two voltages, one taken from an intermediate amplifier stage and a` variable cathode voltage taken from a carrier frequency stage.

We shall describe one form o f apparatus, embodying our invention and shall then point out the novel features` thereof in claims.

The accompanying drawing is a diagrammatic view showing one form of apparatus, embodying our invention when used with a superheterodyne type of receiving apparatus for an inductive carrier railway train telephone system employing frequency modulation. It is to be understood` that our invention is not limited in its` use to train telephone receiving, apparatus,` and this one application of the inventionserves to illustrate the many places the apparatus is useful Referring to the drawing the receiving apparatus is of the superheterodyne, type for frequency modulated carrier telephone c-urrentand it includes a first and a second carrier` frequency amplifier stage, a converter stage, an intermediate frequency amplifier stage, a limiter and a discriminator, together with audio frequency stages as required.V

The reference character PC designates a pickup coil or antenna capable of picking up frequency modulating carrier telephone current sent out by a remote transmitter, not shown. lIfhe coil PC is connected to the input of a first carrier frequency amplifier CAL shown in block form, since it may be of standard form and its specific structure forms no part of our invention The output of the amplifier CA I, includes a tuned primary winding `l 0 of a coupling transformer TI, a tuned secondary winding H of which transformer is connected to the input of a second carrier frequency amplifier tube V2, tube V2 having a control grid cathode circuit including secondary winding l I oi transformer TI ground, a manually adjustable potentiometer NS and a resistor Rt. An anodecathode circuit of tube V2 is pow- -tiple with resistors RI 6 and R550 in series.

ered from positive terminal B300 of a suitable power source, the negative terminal of which source is connected to ground. in the usual manner for the power supply of such receiving apparatus. This anode cathode circuit is completed through the input of a converter stage HD.

The converter or mixer stage HD is shown in block form since such device may be any one of several well-known arrangements, and it is suficient for the present application to point out that the amplied frequency modulated telephone current is heterodyned with a local generated carrier to provide an intermediate carrier wave having the modulation of the original received Wave. The output current of the converter stage HD is supplied to a tuned primary winding I2 of a coupling transformer T2, a tuned secondary winding I3 of which transformer is connected to the control grid cathode circuit of tube V4 of the intermediate frequency amplifier. This control grid cathode circuit for tube V4 is completed through resistors RIZ, R 59 and RI3, the junction `terminal of resistors R59 and RI 3 being connected to ground. An anode cathode circuit for tube V4 is powered from the B35!! source and includes a, tuned primary winding I4 of a coupler transformer T3, a tuned secondary winding I5 of which transformer is connected to an input or control grid cathode circuit of a limiter tube V5. This .circuit for tube V5 is completed through a biasing unit BU which comprises a capacitor I6 in mul- In the usual manner the limiter tube and its associated circuits are arranged for its anode circuit to saturate with a small positive control grid Voltage so that the amplitude of the output is substantially constant for all control voltages above a minimum required value. The output of limiter tube V5 is supplied to a discriminator network DS.

The discriminator network DS is shown in block form since the details of such network are well known and it is suiiicient for this application to point out that the constant amplitude varying frequency wave of the limiter tube output when applied to the discriminator is converted into a modulated rectied wave of constant carrier frequency and varying amplitude.

The output of the discriminator DS is applied to the input of a rst section of a twin triode .tube VTI, and which section serves as a. phase inverter. The second or right-hand section of tube V1 as viewed in the drawing serves as a control or squelch tube to be more fully explained shortly. The tWo sections of tube VI are coupled to a twin triode controlled amplifier tube V8, the input of the two sections of tube V8 being arranged in such a manner that the communication energy from the phase inverter section of tube VV'I is applied in push-pull and a voltage from the squelch section of tube VI is applied in parallel to the grids of tube V8. Specificially, the anode and cathode of the phase inverter section 0f tube V'I are connected to the grids of the two sections of tube V8 through capacitors II and I8, respectively, and the anode circuit of the squelch section of tube VI is connected to the grids of. tube V8 through resistors R34 and R35. respectively. 'I'he anode circuit of the phase inverter section of tube V7 is powered from the terminal B350 through resistors R21 and R23 in series, and the anode circuit of the squelch section of tube V1 is powered from a voltage divider including resistors R3! and R32 connected across the power source through another resistor R35, the junction terminal of resistors RSI and R32 being connect- 4 ed to the anode of the squelch section of tube V1 through a resistor R33 and the cathode of the squelch section being connected to ground.

The control or squelch section of tube V'I is normally conductive for reasons to appear hereinafter, and its anode current drawn through resistor R33 of a relatively high value drops the anode voltage to a relatively low value and therefore the voltage applied through resistors R34 and R35 to the grids of the controlled tube V8 is of a 10W value. The cathodes of the tube V8 are connected to the voltage divider in a manner to have a relatively high potential due to the voltage divider effect of resistors R36, RSI and R32. In this Way the controlled tube V8 is biased to cutoi and is desensitized as long as the squelch section of tube VI is conductive. With the controlled tube V8 desensitized then no energy is passed to an audio amplifier stage AA and on to a loud-speaker LS. That is to say, the output of the two sections of the controlled tube V3 are connected to an audio amplier AA in a pushpull manner, the audio frequency amplifier AA being shown in block form since it may be of any standard arrangement. The output of the amplifier AA is used in turn to supply energy to a telephone receiver such as a loud-speaker LS. It is to be seen therefore that as long as the squelch tube is conductive the controlled tube V8 is maintained desensitized and no energy is passed to the audio amplifier and on to the loud-speaker.

The noise responsive circuit network includes a noise amplier tube VII, rectiers I9 and 20, and an automatic volume control circuit together with the squelch section of tube V'I.

It is believed that the noise responsive circuit network and its connections can best be understood from a description of the operation of the apparatus, and in describing the operation of the apparatus We shall iirst consider the normal stand-by condition, that is, the condition when no communication current is being received. Under these circumstances the apparatus operates at its greatest possible sensitivity so that it will detect communication current of a minimum energy level While at the same time the noise suppression means blocks the passing of noise voltages to the audio amplifier and on to the loudspeaker.

Assuming a noise voltage is picked up by the coil PC, this voltage is ampliiied at the first and second stages of the carrier amplifiers and is impressed upon the local carrier at the converter stage HD and is then applied to the grid circuit of the intermediate frequency amplifier tube V4. This noise energy after amplification at tube V4 is applied to limiter tube V5 and the resultant grid current of tube V5 causes a direct voltage drop across resistors R60 and RIG, the junction terminal A of resistors R60 and RIB being negative in potential with respect to ground. The direct voltage of terminal A is applied to control grid 24 of the squelch section of tube V'I, the connection including resistors R29, R42 and R43. This voltage at terminal A being negative in potential with respect to ground, it tends to render the grid 24 of the squelch tube negative with respect to the cathode and hence tends to render this tube non-conductive.

At the same time, the amplitude variations of the noise voltage at terminal B of the grid circuit of the limiter tube V5 is applied through resistor R36 and capacitor 25 to a. control grid 2I of noise amplier tube VII, and such noise voltage is 7l amplified in the anode circuit of the tube VII.

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esegesi Thealternating current component 'of the anode `circuit of tube VII is applied to rectiiiers I8 `and 20 through e, couplingicapacitor 2,1. *Ihe output sides of the rectiers I9 and 2i) are connected to `two capacitors 28 :and `2.9, the connection being such that the two capacitorsare -charged alternately. The two capacitors `28 land 29 in series are connected across resistor R42 :as `a load resistor and develop a voltageiacross this resistor which is applied through resistor R49 to `thecon trol grid 24 of the squelch tube. The polarity of this `voltage developed across resistor yR42 is such as `to drive thesquelch tube inthe positive direction or `in other words, thepolarityof the second direct voltage applied to `the squelch tube "is opposite that applied `to :the -squelch tube from terminal A of the .fgrid circuit `of limiter `tube V5. The parts are so `proportioned that these two direct `voltages when produced by -noise `energy `balance out and the squelch tube remains con-l ductive, or in other words, its conduction is not disturbed by the noise ienergy andas long 'as the squelch `tube remains `conductive then the controlled `tube V8 remains `desensitized `and the noise energy is not passed to the audio amplier.

`It is to Vbe 'pointed out `that when this noise energy is applied to the intermediate frequency lamplier tube V4, a direct `current "ovvs in the `grid circuit and `a direct voltage is ldeveloped across theccmbination of resistors R59 and R12,

the junction 'terminal `D 4of `the two resistors Ibeing `negative with respect to ground. This direct voltage at terminal `D is applied to `the `control grid 2I of `the Anoise amplifier tube VII, through `a connection including `resistor R57, and since it is `negative with vrespect 'to ground lit tends to decrease `the gain at .the lnoise amplifier tube, orin other words, it tendsto act as an automatic `volume ccntrolvoltage` for the tube V-'I I.

There is also a `biasvoltage appliedlto cathode- 23 of thenoise amplifier tube VII 1by means of a connection including resistors R24 `and R4 and the manually adjustable potentiometer NS, as will be readily understood by an inspection of the drawing. This latter voltage is a combination of that developed by the cathode current of tube VI I flowing through resistors R24, R4 and NS, plus that developed by the cathode current of tube V2 flowing through resistors R4 and NS. As pointed out hereinbefore, the potentiometer NS is manually set for the most sensitive condition of the tube V2 and `under such conditions the gain of the noise amplifier VII is a function of the voltage developed at terminal D.

Due to the automatic volume controlaction on A tube VI I by the noise voltage derived at terminal D of the grid circuit of tube V4, the `tvvo `noise voltages applied to the grid 24 of the squelch tube are made substantially equal and produce a resultant voltage of approximately zero through a relatively wide range of noise energy level, with the result that the function of the squelch tube is not disturbed or varied by noise energy of a relatively wide range. That is to say, if the noise voltage picked up is very high, the amplicatiou of the voltage may be almost linear up to the grid circuit of the limiter tube and grid current of tube V4 will produce a corresponding voltage drop across resistor R59 with the result that a high negative voltage of terminal D is applied to the grid of the noise amplifier tube VII to reduce its gain. The direct voltage across resistor RBD will not increase to such a great extent because tube V5 acts as a limiter tube. The alternating current voltage at terminal B `would increase in proportion 4.to the Ainput noise `voltage but lwould not be ampli-fled to such a great extentby tube VII due `to the automatic volume `control action and the `voltages applied to the `squelch tube through resistor R49 will remain Aapproximately equal and opposite.

We shall next 'assume that frequency modulated carrier telephone current for which the apparatus is designed is 4picked up by the coil PC. This telephone current is amplied through the various stages and a corresponding direct voltage is developedacross resistor R59 due to the grid current produced for the tube V4. This voltage of terminal D'isapplied to the noise ampliiier tubeVII `to reduce its gainas explained hereinbefore. The 'telephone current also produces a direct voltage at terminal A of `the limiter tube. "the `voltage of terminal A now being that produced not only by the carrier telephone current 4but also by any noise voltage that maybe present. This direct voltage at terminal A'is applied to the control grid 24 of lthe squelch tube tending to cnder that squelch tube non-conductive. The telephone current will produce little or no alterhating voltage at terminal B to be applied `to the noise amplier tube VI I. Thus the voltage created across the resistor R42 due to Jthe output of the noise amplifier .tube VI I is not increasedby the telephone current energy. rIhis means that the resultant voltage applied to grid 24 of .the squelch tube is negative in value and the squelch tube is made non-conductive. The negative bias voltage applied to the squelch tube is ordinarily of a value sufcient even with a very Weak carrier telephone current to cause the squelch tube to become substantially non-conductive. When anode current `of the squelch .tube ceases to flow through resistor R33 .of .the voltage divider `network, the negative bias of the controlled tube V8 is removed `to a degree that the tube V8 functions at substantially its usable sensitivity, with the result the telephone current energy is passed to the audio amplier AA `and the speech is sounded at the loud-speaker.

We have 'found that noise suppression means lsuch as here disclosed and using an automatic volume control for the noise amplifier can `,be adjusted to substantially suppress noise of re1- -atively large amplitude and ,yet be responsive to very Weak telephone currents.

Although we have herein shown and described but one form of noise suppression means for communication receiving apparatus embodying our invention, it is understood that various changes and modifications may be made therein `within the scope of the appended claims without departingfrom the spirit and scope `of our invention.

Having thus described our invention, what We claim is:

1. In noise suppression means for receiving apparatus using a modulated carrier current and which apparatus includes a carrier frequency amplier tube, an intermediate frequency .amplifier tube, a limiter tube, a demodulator and a controlled audio frequency amplifier tube and each of which tubes is provided with an anode, a cathode and a control grid; the combination comprising, a control tube having an anode, a cathode and a control grid, said control tube having an anode-cathode circuit, means including a coupling between the anode-cathode circuit of the control tube and the control grid-cathode circuit of the controlled audio frequency amplifier re sponsive to the conduction or non-conduction of Vaccessi -t-he control tube to render the controlled tube insensitive or sensitive, a control grid-cathode circuit for said limiter tube and which circuit includes a control unit comprising resistance and capacitance connected in multiple, said unit being connected in the control grid-cathode circuit of the limiter tube adjacent the cathode, a first and a second circuit means t connect said control grid-cathode of said limiter tube to the control grid of said control tube, said first circuit means including resistance and connected between an intermediate terminal of said resistance of said control unit and the control grid of the control tube, said first circuit means being effective to apply to the control tube a direct voltage due to the limiter tube grid-cathode rectification all energy picked up by the receiving apparatus and ,which direct voltage is of the polarity which tends to render the control tube nonconductive; a noise amplifier tube having an anode, a cathode and a control grid; a rectifier, said second circuit means including in cascade said noise amplifier tube and said rectifier and connected between a terminal cf the capacitance of said control unit and the control grid of the control tube, said second,

circuit means being effective to apply another direct voltage to the control tube due to the amplified and rectified alternating current component of noise energy applied to the limiter tube and which last mentioned direct voltage is of a polarity which tends to render said control tube conductive, a resistance capacitance unit interposed in a grid-cathode circuit of said intermediate frequency amplifier tube, an adjustable resistorV interposed in the cathode lead of said carrier frequency ampliiier tube, and a third circuit means having a first path including a resistor to connect the grid of the noise amplifier tube to the grid circuit unit of said intermediate amplifier :tube to apply a direct voltage developed by reati-- of the noise amplier tube and a second path including another resistor to connect the cathode of the noise amplifier tube to said adjustable resister which is interposed in the lead of the cathode of said carrier frequency amplifier tube to select the sensitivity of said noise amplifier tube in step with the adjustment of the sensitivity of the carrier frequency amplifier tube.

2. In noise suppression means for communication current receiving apparatus which includes a carrier frequency amplier tube, a converter, an intermediate frequency amplifier tube, a limiter tube, a discriminator and a controlled audio frequency amplifier tube and each of which tubes has an anode, a cathode and a control grid l and each is provided with an anode-cathode circuit and a grid-cathode circuit, the grid-cathode circuit of the intermediate amplifier tube and of the limited tube including a resistance capacitance unit and the anode-cathode circuit of the carrier frequency amplifier tube including an adjustable resistor interposed in the cathode lead of the circuit; the combination comprising; a control tube having an anode, a cathode and a control grid and provided with an anode-cathode circuit,

means including a coupling between the anodecathode circuit of the control tube and the grid-cathode circuit of said controlled audio frequency amplifier tube responsive to the conduction and non-conduction of the control tube to render the controlled tube insensitive or sensitive: a noise amplifier tube having an anode, a

vcathode and a control grid; a rectifier, circuit means including two paths connected between said unit of the limiter tube grid-cathode circuit and the control grid of said control tube to control the conductive condition of the control tube, a first one of said paths including resistance only to apply a direct voltage developed in the gridcathode circuit of the limiter tube due to the grid rectification of energy applied to the limiter tube, a second one of said paths including said noise amplifier tube and said rectifier to apply a direct voltage due to the amplifying and rectifying of the amplitude variations of the energy applied to the limiter tube, said noise amplifier tube provided with an anode-cathode circuit and the cathode lead of which includes said adjustable resistor for the sensitivity of the noise amplier tube 4to be selected according to the selected sensivtivity of the carrier amplifier tube, and means to connect the control grid of the noise amplifier tube to said unit in the grid-cathode circuit of the intermediate amplifier tube to automatically control the gain of the noise amplifier tube due to a direct voltage developed by the grid rectiiication of energy applied to the intermediate amplifier tube.

JOHN E. WERNER.

HAROLD D. BECHTOL.

REFERENCES CITED The following references are of record in the file of this patent:

'UN'riED STATES PATENTS Number Name Date 2,101,549 Lamb Dec. 7, 1937 2,239,901 Percival Apr. 29, 1941 2,341,325 Kenefake Feb. 8, 1944 2,343,115 Noble Feb. 29, 1944 2,372,934 Campbell Apr. 3, 1945 Certificate of Correction Patent No. 2,509,381 Mey 30, 1950 JOHN E. WERNER ET AL.

It is hereby certied that errors appear in the printed specication of the above numbered patent requiring correction es follows:

u Columnf', line 9, after grid-cathode insert the Word circuit; line 16, before e insert o end that the seid Letters Patent should be reed with these corrections therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 29th day of August, A. D. 1950.

THOMAS F. MURPHY,

Assistant 'ommasz'oner of Patenta. 

